It has long been known that the properties of thermoplastic resins can be improved in certain respects by incorporating certain fibrous materials into the resins. For example, it has been suggested that the thermal behavior and sliding friction of such thermoplastics may be modified and improved by incorporation of a single type of fiber.
It has also been suggested in German Preliminary Patent Publication 2,016,746, published on Oct. 22, 1970 that various fibers of carbon or glass can be added to a thermoplastic resin, but without suggesting that any critical proportions of the materials are required to achieve an improvement in such properties as bending strength, tensile strength, heat resistance or elastic deformability.
Another material intended for use in friction bearings is described in German Preliminary Patent Publication 2,433,401 published on Jan. 30, 1975. This reference suggests various possible additives, including mixtures thereof, which should improve the properties of the material especially at room temperature.
However, neither of these references suggests additive materials or proportions thereof which would provide the properties necessary to produce satisfactory roller bearing cages which demand impact strength, bending strength, tensile strength, heat and age resistance and elastic deformability of a higher order than required for the friction bearings for which the materials of the prior art were intended.
As those skilled in the art are aware, the conditions of use of roller bearing cages require that they be composed of a material having substantial impact strength or toughness, tensile strength, elastic deformability and aging resistance, greater than those provided by the materials of the prior art. The molding materials of the prior art have included thermoplastic resins reinforced with carbon and glass fibers which have been known to improve the bending strength, tensile strength, heat resistance and term of use of friction and other bearings. However, there has been no suggestion in the prior art of how to improve the elastic deformability of these materials which is so necessary to their use in roller bearing cages. Indeed, it has been observed that the elastic deformability of the molded resins decreases with increasing content of fiber materials. For this reason, it has been found that elastically deformed parts of roller bearing cages made from such materials, such as retaining shoulders and the like, break off immediately or at best after only a short operating period causing the roller bearing to fail prematurely. This has been found to be especially the case with carbon fiber reinforced plastics in which other properties such as heat resistance and shape retention are improved as a result of the high fiber content.
Elastic deformability is an especially important characteristic in parts produced by injection molding since complex tools must be constructed to permit the application of known construction materials to produce molded parts having the structural strength and properties required of roller bearing cages or similar parts. Such tools must be removed first in the construction of multi-slides, for example, particularly from undercut recesses in injection molded parts, such as pockets in roller bearing cages with retaining shoulders, before removing the work piece from the mold, and this, of course, requires a high degree of elastic deformability to avoid breakage of the work piece.
Another problem of the prior art has been that the previously preferred known molding compositions used in the manufacture of roller bearing cages have had insufficient resistance to aging.
Still another problem has been that articles molded from the compositions of the prior art had too little impact toughness and were easily broken in use.
Moreover, conflicting requirements must often be met in selecting a molding composition for use in the manufacture of roller bearing cages, for example, thus making it necessary to compromise properties when using prior art compositions.
It is apparent, therefore, that a need has existed in the art for an improved thermoplastic molding composition possessing the combination of properties required to produce shaped parts, such as roller bearing cages, having in the molded and set article, the combination of impact strength, bending strength, tensile strength, heat resistance, aging resistance and elastic deformability required to ensure reliable performance and long operating life of the article in question.
It is, therefore, an object of the present invention to provide a thermoplastic molding composition which is inexpensive and readily processed to produce molded and shaped parts having improved impact strength, or toughness, which are deformable within predetermined limits, and which are also resistant to aging, even at relatively high operating temperatures and under adverse operating conditions under the effect of foreign substances and retain normal values or have improved values of bending strength, tensile strength, coefficient of heat expansion, wear factor, coefficient of friction, and heat conductivity.
It is a particular object of the invention to provide a thermoplastic molding material especially suitable for the production of roller bearing cages.